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ORLANDO, Fla. — If humans ever build an interstellar spaceship —a
vehicle capable of reaching another star — one of the biggest
questions will be which of the billions of stars in the Milky Way
should it visit?

Scientists debated possible interstellar destinations at the
100-Year Starship Symposium, a weekend meeting here sponsored
by the Defense Advanced Research Projects Agency (DARPA) to
discuss planning the first mission to another star system.

Among the top priorities for choosing a star to target is its
potential to harbor life, said astrobiologist Jill Tarter of the
SETI (Search for Extraterrestrial Intelligence) Institute.
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"It's really the story of
life in the cosmos that is likely to drive exploration beyond
the solar system," Tarter said."I think this is the question that
will be worth the effort and the pain and the investment of
traveling to another star system."

Tarter and other experts agreed that any interstellar mission
should try to visit a star that has planets — hopefully planets
the right size and distance from their stars to host life.

The symposium is part of the 100 Year Starship Study, a $1
million, one-year project of DARPA and NASA to look into what it
would take to launch a mission to another star within a century.
In November, the agencies plan to award $500,000 in seed money to
an organization that can spearhead the effort to research the
necessary technology and logistics.

Having planets isn't the only qualification the chosen star must
meet. Another important criterion is its distance from Earth —
the closer, the better.

At 4.4 light-years away from the sun, Alpha
Centauri is our nearest stellar neighbor, putting it at the
top of the list of candidates. However, even Alpha Centauri would
be a long trip.One light-year, the distance light crosses in a
single year, is about 6 trillion miles (10 trillion kilometers).

To put such a distance in perspective, one speaker at the
symposium suggested an analogy: If the Earth was here in Orlando,
and Alpha Centauri was in Los Angeles, then the entire solar
system would span only 1 mile (1.6 km).

The farthest any manmade object has ever traveled is
approximately to the edge of the solar system. That object, the
Voyager probe, is traveling 38,000 mph (61,000 kph). While that
might sound speedy, it's no match for interstellar distances.

For a true interstellar mission, scientists will have to develop
new means of propulsion, such as nuclear-powered engines.

Project Icarus

One group working on the problem is
Project Icarus, a joint endeavor by the Tau Zero Foundation
and the British Interplanetary Society, to design an interstellar
spacecraft. This first potential mission would not carry humans
onboard, but would aim to send robotic probes to investigate a
nearby star and its environs.

"The way to view it would be an incremental approach," said
Icarus designer Ian Crawford, a planetary scientist at Birkbeck,
University of London. "We would probably stick with a closer
target initially to develop the expertise of interstellar
spaceflight. Then you can move on to more distant targets. I
don't think it's wise to bite off more than you can chew
initially."

Project Icarus has chosen to give itself a deadline of 100 years,
meaning the spacecraft must be able to reach its destination
within a century, preferably sooner, of launching. [ Most
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The Icarus designers are focusing on building a nuclear-powered
spacecraft, which they hope would be able to travel at up to 15
percent the speed of light (light travels at 186,000 miles per
second, or 300 million meters per second). At that rate, the
farthest the Icarus spaceship could reach in 100 years would be
about 15 light-years away.

Within 15 light-years of the sun, there are 58 known stars in 38
separate stellar systems (many of the stars are in binary pairs).
Of those 58 stars, two are currently known to have planets.

However, because extraterrestrial
planet-hunting is just heating up, scientists expect many of
the others also host planets that just happen to be currently
undetectable.

However, even the two nearest stars known to have planets are
still good initial candidates.

One is called Epsilon Eridani, and lies 10.5 light-years away. It
is known to have a planet that weighs about 1 1/2 times the mass
of Jupiter, and also has a disk of dust around it that suggests
there are likely other, smaller planets present, too.

The second candidate is called GJ 674. At 14.8 light-years away,
this one is pushing right up against the 15-light-year limit, but
it could still be a viable option, experts said.

And don't forget Alpha Centauri. Though no planets have yet been
discovered around this star, that doesn't mean there aren't any.

"My own view is that Alpha Centauri will only lose its place at
the top of the list if we determine that it doesn't have a
planetary system," Crawford said.

The science of detecting alien planets is developing quickly, and
researchers will probably have a much better idea where the
nearby planets are by the time the first interstellar spaceship
is ready to fly.

"The main thing to note here is that long before we can build an
Icarus vehicle, astronomical tools will have told us where the
planets are orbiting nearby stars," Crawford said. "Within 100
years, I'm reasonably confident we'll have a complete inventory.
I think the take-home message here is, by the time we're ready to
build an interstellar vehicle, we will in fact know where to send
it."

You can follow SPACE.com senior writer Clara Moskowitz on
Twitter @ClaraMoskowitz.
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